1. Field of the Invention
The present invention relates to an anisotropic bonded magnet which is made by compression molding to obtain a high density and a high magnetic powder alignment and also relates to its production method.
2. Description of the Related Art
Bonded magnets are made of magnet powders embedded in organic resin or metallic resin. They have lower levels of magnetic energy compared to their fully densified counterparts such as sintered magnets.
Bonded magnets, which have excellent formability, can be formed in complex shapes with close mechanical tolerances as well as are free from cracking. Because of the above mentioned advantages, their application area is rapidly spreading.
They may be formed by extrusion, compression, and injection molding.
Injection molding has advantages in forming complex shapes and integrated components with high precision. Injection molding can form the most complicated shapes among three mentioned molding methods. However the magnet has low magnetic energy because the volume fraction of magnet powder is limited to under 60 to 65 percent to provide good flowability in the process.
Extrusion molding has merit during continuous production to provides a low cost magnet. Also extrusion molding gives better magnetic energy than injection molding due to the volume fraction of magnet powder of 70 to 75 percent.
Compression molding gives the highest magnetic energy because of maximum volume fraction of 80 to 90 percent. It can also produce magnets of complex shape.
As mentioned previously, the major shortcoming of the bonded magnet is its low maximum energy product. Recently anisotropic magnet powders with high maximum energy product have been developed to overcome the mentioned shortcomings. However compression molding which is suitable for anisotropic magnet powder has not been established facing the difficulty of contradiction of high density and magnet powder alignment.
Here, anisotropic magnet powder is an aggregation of fine magnet particles which consist of uniaxial crystals and have unidirectional magnetization. Magnet powder alignment means to align the magnetization of each particle to an applied magnetic field.
To solve the difficulty described above, the following technology concerning compression molding has been proposed.
In Japanese patent application Laid- Open (Kokai) No. 1-205403, warm molding is proposed in order to utilize a resin softening phenomenon before curing reaction is set forth. The compound consists of melt spun magnet powder and thermosetting resin.
The patent application discloses that high density is attained by use of a heating temperature of 30 to 100.degree. C. at which the softening phenomenon occurs before curing reaction. Above 100.degree. C. curing has begun before the magnet was sufficiently densified. Further, adhesion of resin to the mold is observed.
Using a compound which consists of Nd.sub.14 Fe.sub.76 Co.sub.5 B.sub.5 melt spun powder and epoxy resin, a bonded magnet with (BH)max of 10.3-11.2 MGOe and density of 6.7 to 7.1 g/cm.sup.3 was attained by compression molding at 45 to 77.degree. C.
However the magnet shows a relatively low maximum energy product of 11.2 MGOe in spite of high density of 7.1 g/cm.sup.3, because the melt spun rare earth magnet powder is isotropic.
Furthermore, the applied resin shows poor thermal durability due to its low melting point.
In Japanese patent application Laid- Open (Kokai) No. 2-116104, compression molding below curing temperature followed by curing is proposed. The molding temperature is set between softening point and about 50.degree. C. higher in order to increase flowability of the resin. In this embodiment a bonded magnet with maximum energy product of 9.0 MGOe and density of 6.1 g/cm.sup.3 was obtained.
A compound consists of ortho-cresol novolak type epoxy resin with a melting point of 40.degree. C. and rare earth magnet powder was compression-molded at 100.degree. C. followed by curing at 120.degree. C. The patent application discloses that the magnet which has superior thermal durability and relatively high magnetic property with high density are obtained by forming at an elevated temperature to keep the softening state of the resin. However the magnet shows considerably low maximum energy product of 9.0 MGOe because of low density of 6.1 g/cm.sup.3. Furthermore the invention requires a curing process after compression molding.
Japanese patent application Laid- Open (Kokai) No. 4-11702 proposes fine resin powder which enables high magnetic property by means of reducing the amount of resin to that of magnet powder. In the invention compression molding with a magnetic field is disclosed.
The particle size of the magnet powder is from 0.1 to 500 .mu.m, which is ordinary for this kind of use, and the particle size of the resin powder is chosen to be one tenth of the magnet powder. When they are mixed into a compound, fine resin powder covers the surface of magnet powder evenly by electro-static force. In the invention, a bonded magnet is manufactured in a single process, namely, curing is performed at the same time as compression molding. In at least one of the embodiments, a magnetic field of 15,000 Oe is applied in the compression molding.
In embodiment 1, a compound of barium ferrite magnet powder and fine polymethylmethacrylate powder with the particle size of 0.05 to 0.06 .mu.m was compression-molded with the applied magnetic field of 15000 Oe. Curing reaction proceeds simultaneously with the molding. The obtained bonded magnet had the density of 3.40 g/m.sup.3 and (BH)max of 1.35 MGOe.
In embodiment 3, a compound of NdFeB magnet powder (MQ powder A) and fine polymethylmethacrylate powder with the particle size of 0.05 to 0.06 .mu.m was compression-molded with the applied magnetic field of 5000 Oe. Curing reaction proceeds simultaneously with the molding. The magnet had the density of 5.49 g/cm.sup.3 and (BH)max of 7.3 MGOe.
In the embodiments, anisotropic powder such as hexagonal plate shaped barium ferrite or elongated NdFeB powder is used. Applied magnetic field aligns the direction of magnetization of the anisotropic powder and thus increases the maximum energy product of the bonded magnet.
In spite of the statements that the invention provides good magnetic property and high density because of reduced amount of resin, the obtained bonded magnets have low maximum energy product and low density. Therefore this invention is considered rather to combine molding and curing processes into one production step.
Japanese patent application Laid- Open (Kokai) No. 4-349603 proposes micro-capsules made of thermal polymerized resin which contains lubricant. The capsule coats the surface of magnet powder and reduces frictional resistance of the compound at the mold surface to obtain a high densified magnet. Furthermore the capsule avoids galling of mold by the compound.
In embodiment 1, a compound of (Pr, Sm)Co magnet powder are compression-molded with an applied magnetic field of 24 kOe and cured at 180.degree. C. A bonded magnet with maximum energy product of 15.0-15.7 MGOe and density of 6.82-6.95 g/cm.sup.3 was obtained.
However the invented process had not attained sufficient maximum energy product in spite of its high density. It may be due to low degree of magnet powder alignment although compression molding is carried out in a magnetic field of as high as 24 kOe. Another problem in this invention is that the manufacturing process of the compound becomes quite complicated. Furthermore, high density brings less amount of resin as binder and causes brittleness of the bonded magnet as well as low flowability in molding.
The major shortcoming of the bonded magnet is low maximum energy product. Therefore development of suitable compression molding has been anticipated to improve maximum energy product. The problem in compression molding of a bonded magnet was contradiction of high density and magnet powder alignment. The problem has not been solved in spite of various inventions as described previously.